(1) Field of the Invention
This invention relates to a radiographic apparatus using a flat panel radiation detector, and more particularly to a technique for removing scattered radiation.
(2) Description of the Related Art
Description will be made taking X-rays as an example of radiation. In many X-ray imaging apparatus, a scattered X-ray removing grid (scattered radiation removing device) is used to prevent a lowering of image quality due to scattered X-rays (hereinafter referred to simply as “scattered rays”) generated when X-rays are transmitted through a subject to be imaged. An ordinary grid is formed of an alternate arrangement of absorbing foil strips which absorb the scattered rays and intermediate layers which transmit the X-rays. However, when such a grid is used, the structure of the grid (absorbing foil strips) will be projected to as shadows. It is therefore necessary to remove the shadows of the grid.
A commonly used grid has a fine structure, and the shadows of the grid are removed by spatial frequency processing or the like. The shadows of the grid can be removed by developing data to a frequency domain by Fourier transform, and restoring the data by inverse Fourier transform after removing specific grid frequencies (see Japanese Unexamined Patent Publication No. 2005-052553, Japanese Unexamined Patent Publication No. 2001-346795 and Japanese Unexamined Patent Publication H11-285493, for example).
An air grid having a higher X-ray transmittance than the commonly used grid has been devised recently. The air grid has voids serving as the above-mentioned intermediate layers, which easily transmit X-rays compared with the intermediate layers formed of aluminum, an organic material or the like. However, this air grid has a larger internal structure than the usual grid, which makes it difficult to remove the shadows by the above-noted spatial frequency processing.
Then, it is necessary to remove the shadows in real space, instead of using the Fourier transform or other measure. The principle of removing the shadows in real space will be described with reference to FIG. 3. As shown in FIG. 3, an air grid 6 has absorbing foil strips 6a arranged with intermediate layers 6c which are voids inserted in between. The absorbing foil strips 6a are thin metallic foil strips formed of lead, for example. The air grid 6 is disposed adjacent an incidence plane of a flat panel (two-dimensional) X-ray detector (FPD) with detecting elements arranged in rows and columns (i.e. in a two-dimensional matrix form). The direction of arrangement of the absorbing foil strips 6a is parallel to the rows of the detecting elements. Spacing between adjacent shadows is larger than spacing of pixels forming an X-ray image.
The absorbing foil strips 6a have a sufficiently smaller width than the pixels. The material used for these foil strips has a very low X-ray transmittance. Therefore, the X-ray transmittance can be expressed by a ratio in the following equation (1), using the area of a portion without shadows and the area of a portion with shadows (area of shadows):X-ray transmittance=(pixel area−shadow area)/pixel area   (1)
In the above equation (1), the numerator (pixel area−shadow area) at the right side is the area of the portion without shadows.
A method of removing the shadows of the air grid 6 (shadows of the absorbing foil strips 6a) may assume that the shadow portion also has received incident X-rays of luminance of the non-shadow portion (portion without shadows) of the same pixel. Therefore, a pixel value after shadow removal by dividing a pixel value (in the shadow portion) at the time of image pickup by an X-ray transmittance at the pixel concerned, as in the following equation (2):pixel value after shadow removal=pixel value at image pickup time/X-ray transmittance   (2)
Thus, the shadows can be removed by first deriving an X-ray transmittance from equation (1) above, and deriving a pixel value after shadow removal from equation (2) above using the X-ray transmittance and the pixel value at the time of image pickup.
In the principle of removing the shadows in real space, the shadows are removed using what is shown in equation (1) above. In practice, an image of the air grid alone is picked up in advance of X-ray imaging. That is, grid data is first acquired by X-raying the air grid without a subject to be imaged (i.e. in the presence of the air grid only), and thereafter image pick-up data is acquired by carrying out actual X-raying in the presence of the subject to be imaged and the air grid.
The grid data is acquired in advance of actual X-ray imaging, and X-ray transmittances are derived from the grid data by the equation (1) above. However, such X-ray transmittances cannot be applied directly to actual image pick-up data by using equation (2) above. The reasons are as follows:
(A) In some cases, a shadow straddles a plurality of pixels. Specifically, as shown in FIG. 14A, a shadow 32 straddles two adjacent pixels 31. When a movement of the focal position of (the vessel) of an X-ray tube moves also the shadow 32 as shown in FIG. 14B, the quantities of the shadow 32 on the pixels 31 will change. The focal position of the X-ray tube is changeable because, in spite of the condition that the X-ray focal point of the X-ray tube, the grid and the FPD ought to be in a fixed relationship, when the X-ray tube, FPD, etc. are moved together, the movement will cause a shifting of the positional relationship between the X-ray focal point, grid and FPD.
For example, when this image pickup unit is applied to an actual medical apparatus, such as an apparatus used for cardiovascular diagnosis (CVS: cardiovascular system), a C-arm is usually used to conduct diagnosis (that is, X-ray image pickup). The C-arm literally has a curved shape of character “C”. The C-arm supports a radiation emitting device such as an X-ray tube at one end thereof, and an FPD at the other end. When the C-arm is rotated along the direction of its curve, the X-ray tube and FPD revolve with this rotation. During such movement, X-raying is carried out with the X-ray tube emitting X-rays, and the FPD detecting the X-rays. At the time of picking up an image of a patient, in spite of the condition that the X-ray focal point of the X-ray tube, the grid and the FPD ought to be in a fixed relationship, the rotation of the C-arm and the like will cause a shifting of the positional relationship between the X-ray focal point, grid and FPD. Especially, the shifting of the positional relationship between the X-ray focal point, grid and FPD easily occurs under the weight of the FPD and
X-ray tube. It is not realistic from the hardware point of view to fix the positional relationship.
(B) When imaging a human body such as a patient, it is impossible to carry out inverse calculations from a ratio of shadows in grid data obtained only from the grid as a result of part of scattered rays from the patient passing through the grid. Calculations based on the ratio are possible only with part of direct X-rays (hereinafter referred to simply as “direct rays”).